Electrocardiographic techniques provide graphic data regarding cardiac events using a number of electrodes placed in electrical contact with a patient's skin. The electrodes are connected to an electrical device which receives signals via the electrodes, amplifies them, and presents them typically on graph paper via electromechanical means. Typical present day electrocardiographic devices use hot filaments to burn a track on a strip of heat sensitive paper as the paper moves at constant speed. The filament is deflected up or down in accordance with variations of the signals associated with cardiac events.
Electrocardiographic paper is printed with a grid of vertical and horizontal lines at intervals of one millimeter, the distance between vertical lines representing four hundredths (0.04) second. Every fifth line is heavier, thereby making intervals of one-fifth (0.2) second. The paper is transported at a standard speed of 25 millimeters per second, the equivalent of 150 centimeters per minute. Horizontal distances on the paper thus represent time, and are generally referred to in units of time.
Cardiac events are obviously independent of the paper movement, and in any event typically do not exhibit the periodicity of the grid lines, so that graphic information transcribed on the paper (the electrocardiogram) is interpreted using the horizontal and vertical lines only as a gauge for estimation.
As shown in FIG. 1, a normal cardiac cycle as presented on an electrocardiogram consists, from left to right, of (1) a "P wave" associated with atrial contraction, (2) an interval of electrical neutrality, (3) the "QRS complex" associated with ventricular contraction, (4) another interval of neutrality, (5) the "T wave" associated with ventricular recovery, and (6) a period of neutrality ending when a successive "P wave" starts a new cycle. The term "R-R interval" is used to refer to the distance from a prominent point in a cycle to the corresponding point in the succeeding cycle, and, with normal regular rhythm, is equal in length to a cardiac cycle, as shown in FIG. 1. As generally accepted, the range of normal cardiac cycle frequency or cardiac rate extends from 60 to 100. The term "bradycardia" refers to cardiac rates below 60, and "tachycardia" refers to cardiac rates greater then 100.
Basic ECG interpretation techniques include the following:
(1) Measurement of cardiac rate in cycles per minute;
(2) Measurement of the PR interval (time from the beginning of the P wave to the beginning of the QRS complex);
(3) Measurement of the duration of the QRS complex; and
(4) Measurement of the QT interval (time from the beginning of the QRS complex to the end of the T wave).
Prolongation of the PR interval or the QT interval beyond accepted standards for age and sex indicates disease. For example, a prolonged PR interval in a child may indicate rheumatic fever.
Conventional interpretation of electrocardiograms involves measurement of the duration of these intervals using the vertical lines of the printed electrocardiographic paper grid. Specially calibrated and generally opaque rulers are also conventionally employed. The intervals which are read using the grid or rulers may be compared with tables which represent a significant sample of patients, both with and without heart disease. The tables are typically arranged by age and (sometimes) by sex for normal and diseased hearts. Tables prepared by various authorities obviously vary. Two such tables, one for upper normal PR intervals and one for upper normal QT intervals, were prepared by R. Ashman and E. Hull and appear in their book, Essentials of Electrocardiography, 2nd edition, McMillan Company, New York, 1947, as reproduced in The Electrocardiographic Test Book, published by the American Heart Association in 1958, both of which are incorporated by this reference. Those tables are as follows:
TABLE 1 ______________________________________ Upper Limits of Normal for QT Intervals Men and RATE Children Women ______________________________________ 40 0.49 0.50 43 0.48 0.49 46 0.47 0.48 48 0.46 0.47 50 0.45 0.46 52 0.45 0.46 55 0.44 0.45 40 0.49 0.50 43 0.48 0.49 57 0.43 0.44 60 0.42 0.43 63 0.41 0.42 67 0.40 0.42 71 0.39 0.41 75 0.38 0.39 80 0.37 0.38 86 0.36 0.37 93 0.35 0.36 100 0.34 0.35 109 0.33 0.33 120 0.31 0.32 133 0.29 0.30 150 0.28 0.28 172 0.26 0.26 ______________________________________
TABLE 2 ______________________________________ Upper Limits of Normal for PR Intervals RATE &lt;70 71-90 91-110 111-130 &gt;130 ______________________________________ Large adults 0.21 0.20 0.19 0.18 0.17 Small adults 0.20 0.19 0.18 0.17 0.16 Children 14-17 0.19 0.18 0.17 0.16 0.15 Children 7-13 0.18 0.17 0.16 0.15 0.14 Children 1.5-6 0.17 0.165 0.155 0.145 0.135 Children 0-1.5 0.16 0.15 0.145 0.135 0.125 ______________________________________
The Ashman and Hull tables are utilized to prepare a preferred embodiment of age- and, where indicated, sex-specific nomograms according to the present invention.
Electrocardiographic interpretation using the paper grids or rulers, mentally noting the relevant interval and comparing, when necessary, to tabular data necessarily involves inaccuracies and induces error. For example, such devices only provide means for measurement of one value at a time so that, when needed, an interrupting reference must be made to the tables commonly at hand, causing loss of eye contact with the cycle in question on the EKG. Use of opaque rulers or strips also obstructs view of an appreciable portion of the electrocardiogram under interpretation. In any event, measurement of the intervals along an opaque straight edge may require multiple positioning: after a first positioning for rate measurement, a second positioning may be required for measurement of the PR interval, and a third for the QT interval, unless the P wave, the QRS complex, and the T wave are fortuitously well placed in relation to the prepared grid. With marginal values there is the further step or steps of accessing one or more tables for upper normal values according to age and, in some cases, sex. A persistent factor frequently contributing to inaccuracy, when scrutiny without measurement is relied on, is the random relationship of the inscription to the grid on the prepared paper. All of these factors introduce room for error.